Wireless telephones operate in more than one frequency band, for example, Cellular 900 MHz and PCS 1900 MHz. In the past, to operate in more than one frequency band, a separate, narrow band, RF circuit was provided for each of the different bands. The advantage was that each narrow band circuit could be tuned precisely for optimum performance within its tuned frequency band. A disadvantage resided in the relatively large collective sizes of the individual circuits, which restrained the ability to make wireless phones smaller. Further, since the addition of each frequency band required an additional tuned circuit, manufacturing costs were increased. Further, switching among the separate circuits required an external switch, a switch that was external to the RF tuned circuits. An external switch increases manufacturing cost, and operates slowly in an external circuit.
As described in U.S. Pat. No. 3,611,154, a known circuit that is switchable to different frequency bands, for example, UHF and VHF bands, has a local oscillator with a transistor that is biased by a bias voltage applied at a point designated “A” to an LC (inductance, capacitance) resonant circuit connected between the base and collector of the transistor. The resonant circuit is referenced to ground or earth. A varicap diode in the resonant circuit has its capacitance varied by the value of its bias voltage, which allows tuning of the resonant circuit of the local oscillator for resonance with a first frequency band, UHF, for example. To switch to a second frequency band, VHF, for example, a switching diode starts conducting. The switching diode is connected at the junction of two inductors in the LC resonant circuit, and is biased to a conducting state by a switching voltage applied at a point designated “S”. There are DC blocking capacitors between earth and the switching diode, such that when a double throw switch is thrown to apply the switching voltage to the diode, the diode is biased to a conducting state, causing current to flow through one of the inductors to ground. Thereby, one of the two inductors becomes shorted to ground, which tunes the resonant circuit for resonance with the second frequency band.
Disadvantages of the known circuit reside in the double throw switch, which is external to the RF tuned circuit. The RF tuned circuit must be manufactured with the switch as an external component. Further, the switch is slow to operate as it is external to the RF tuned circuit. The known circuit is further disadvantageous as having an ECL biased transistor, which is not suitable for low voltage operation. The double throw switch of the known circuit in the off position is shunted to ground through a resistive load in parallel with a Zener diode, which dissipates current, and which is unsuitable for use in a low voltage device, such as, a dual frequency band, personal communications unit.
Another known circuit switchable to different frequency bands is described in U.S. Pat. No. 4,379,269. The known circuit has an FET transistor in which one gate is supplied by a bias voltage and a second gate is fed with a uniform voltage by an automatic gain control, AGC. A voltage that is used as the bias voltage is divided at a voltage dividing point. A switching voltage is used to switch to different frequency bands. The switching voltage is supplied to a switching diode. The switching diode conducts and shorts an inductor of a resonant circuit to ground. The switching diode feeds the switching voltage at the voltage dividing point, which raises the voltage at the dividing point upon the reception of a high frequency band. Upon selection of a low band a bias voltage is applied to the switching diode, which back biases the diode. The diode ceases to conduct, and the inductor of the resonant circuit is no longer shorted to ground. At the same time, the bias voltage is applied to a voltage dividing circuit and is impressed as the bias voltage upon the first gate of the FET, which tunes the RF circuit to a lower frequency band. The FET has its AGC delayed if the bias voltage to the first gate is lowered. The FET has its AGC advanced if the bias voltage at the first gate is made higher. A disadvantage of the known circuit is its unsuitability for a low voltage application, such as personal communication devices, because the switching diode is a discrete circuit element-requiring significant voltage for its bias, either forward or backward bias. Further, the switching diode is an active device having its own characteristics as a reactive element with capacitance and inductance values that deter the precise tuning of the resonant circuit to different frequency bands.